The invention relates to communication systems. More particularly, the invention relates to estimating a path-loss in a communication system.
Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and WCDMA telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates the UMTS standard. This application focuses on WCDMA systems for simplicity, but it will be understood that the principles described in this application can be implemented in other digital communication systems.
WCDMA is based on direct-sequence spread-spectrum techniques. Two different codes are used for separating base stations and physical channels in the downlink (base-to-terminal) direction. Scrambling codes are pseudo-noise (pn) sequences that are mainly used for separating the base stations or cells from each other. Channelization codes are orthogonal sequences that are used for separating different physical channels (terminals or users) in each cell or under each scrambling code. Since all users share the same radio resource in CDMA systems, it is important that each physical channel does not use more power than necessary. This is achieved by a transmit power control mechanism in which the terminal estimates the signal-to-interference ratio (SIR) for its dedicated physical channel (DPCH), compares the estimated SIR against a reference value, and informs the base station to adjust the base station's transmitted DPCH power to an appropriate level. WCDMA terminology is used here, but it will be appreciated that other systems have corresponding terminology.
FIG. 1 depicts a communication system, such as a WCDMA system, that includes a base station (BS) 100 handling connections with four UEs 1, 2, 3, 4 that each uses downlink (i.e., base-to-UE or forward) and uplink (i.e., UE-to-base or reverse) channels. In the downlink, BS 100 transmits to each UE at a respective power level, and the signals transmitted by BS 100 are spread using orthogonal code words. In the uplink, UE 1-UE 4 transmit to BS 100 at respective power levels. Although not shown, BS 100 also communicates with a radio network controller (RNC), which in turn communicates with a public switched telephone network (PSTN).
The signals transmitted in the exemplary WCDMA system depicted in FIG. 1 can be formed as follows. An information data stream to be transmitted is first multiplied with a channelization code and then the result is multiplied with a scrambling code. The multiplications are usually carried out by exclusive-OR operations, and the information data stream and the scrambling code can have the same or different bit rates. Each information data stream or channel is allocated a unique channelization code, and multiple coded information signals simultaneously modulate a radio-frequency carrier signal. At a UE (or other receiver), the modulated carrier signal is processed to produce an estimate of the original information data stream intended for the receiver. This process is known as demodulation.
Good transmit power control methods are important for WCDMA (and other) communication systems having many transmitters that transmit simultaneously to minimize the mutual interference of such transmitters while assuring high system capacity. Depending upon the system characteristics, power control in such systems can be important for transmission in the uplink, the downlink, or both. To achieve reliable reception of a signal at each UE, the SIR of the received signal should exceed a prescribed threshold for each UE. For example, as shown in FIG. 1, consider the case in which the UEs receive, respectively, four signals on a common WCDMA communication channel. Each of the signals has a corresponding energy level associated with it, namely energy levels E1, E2, E3, and E4, respectively. Also present on the communication channel is a certain level of noise (N). For a given UE 1 to properly receive its intended signal, the ratio between E1 and the aggregate levels of E2, E3, E4, and N must be above the given UE's prescribed threshold SIR.
To improve the SIR of a received signal, the power of the transmitted signal may be increased, depending on the SIR measured at the receiver. Power, however, is an important resource in a WCDMA system. Since different channels are transmitting simultaneously at the same frequency, it is important to keep transmit power levels as low as possible while still maintaining an acceptable error rate to reduce the mutual interference between transmitters.
A UE accesses a base station using the random access procedure using a random access channel (RACH). A RACH is an uplink transport channel characterized by a collision risk and by being transmitted using open loop power control. The RACH procedure and channel is described in the 3rd Generation Partnership project (3GPP) technical specifications 25.211 and 25.214. RACHs are always mapped one-to-one onto physical channels (PRACHs), i.e. there is no physical layer multiplexing of RACHs, and there can only be one RACH Transport Channel (TrCH) and no other TrCH in a RACH Composite Coded Transport Channel (CCTrCH). Service multiplexing is handled by the MAC layer.
Random access transmission is based on a slotted-ALOHA approach with fast acquisition indication combined with power ramping. The RACH transmission consists of two parts, namely preamble transmission and message part transmission. The preamble part is 4096 chips (roughly 1 ms) long (256 repetitions of a signature of length 16 chips) and fits into one access slot. The message part is 10 or 20 ms long and is used either for uplink signaling or for transfer of short user packets in the uplink direction. The RACH message part radio frame is split into 15 slots, each of length 2560 chips. Each slot consists of two parts, a data part to which the RACH transport channel is mapped and a control part that carries Layer 1 control information. The data and control parts are transmitted in parallel. A 10 ms message part includes one message part radio frame, while a 20 ms message part includes two consecutive 10 ms message part radio frames.
A UE accesses a base station using an available RACH by transmitting a series of access request preambles with increasing power levels, until the base station detects the access request. That is, the UE attempts to access the base station receiver by using a “power ramping” process that increases the power level of each successive transmitted preamble symbol, as shown in FIG. 2. Referring to FIG. 2, the UE transmits (and re-transmits) the access request preambles with increasing power levels until the base station acknowledges (ACK) that it has received the preamble or a denial of service reply (NACK) is received (“no reply” denotes no message transmitted). As soon as an access request preamble is detected, the base station activates a closed loop power control circuit, which functions to control the UE's transmitted power level in order to keep the received signal power from the UE at a desired level. The UE then transmits its specific access request data. In response, the base station starts the process of controlling the UE's transmitted power via a downlink channel. Once the initial “handshaking” between the UE and base station has been completed, the UE transmits a random access message.
A UE has to determine how much random access transmission power to use initially. Ideally, a UE should select a transmission power level so that an access request preamble is received at the base station with precisely the power needed for correct decoding of the random access message. However, for numerous reasons, it is virtually impossible to ensure that this will be the case. For example, the power of the received signal as required at the base station is not constant but can vary (e.g., due to variations in the radio channel characteristics and the speed of the UE). As such, these variations are to some extent unpredictable and thus unknown to the UE. Also, there can be significant errors due to the uplink path-loss. Path-loss is the signal attenuation occurring over the medium between transmission (TX) and reception (RX) due to several factors, such as distance, fading, interference, canceling multi-path reflections off buildings or other obstructions, and other factors.
Consequently, for the above-described reasons, there is a significant risk that a random access transmission will be received at the base station with too much power. This condition causes excessive interference for other users and thus reduces the capacity of the system. For the same reasons, there is also a risk that a random access transmission will be transmitted with too little power. This condition makes it impossible for the base station to detect and decode the transmission.
The setting of the initial power level is important for a number of reasons, including those discussed above and to avoid the time delay incurred due to the UE re-transmitting the preamble until the base station's acknowledgment message is received, as well as the amount of interference caused by the random access transmission. Currently, the calculation of initial power for sending preambles is based partly on broadcast information and partly on a UE measurement of received signal code power (RSCP). The uplink path-loss is not calculated independently. More particularly, the 3GPP random access procedure is based on the implicit assumption that downlink and uplink path-loss are equal, or their difference is negligible.
The problem with the conventional approach, however, is that there is a significant difference in uplink and downlink path-loss for UEs under a variety of circumstances, and this can be particularly devastating when a UE not moving or is moving very slowly. A need therefore exists for a method and apparatus for determining a difference in uplink and downlink path-loss for setting power levels in UEs, as well as for any other purpose.